1. Field of the Invention
The present invention relates to a crystallized glass which has significantly low specific gravity and high mechanical strength, and also has significantly smooth surface roughness after polishing. The present invention also relates to a crystallized glass substrate for information recording medium, which has physical properties required for substrate for information recording medium of the next generation.
The term “information recording medium” in the present invention means a magnetic recording medium which can be used in hard disks for various electronic devices.
2. Related Art
In recent years, large data files such as movie and audio files have been used in personal computers and various electronic devices, and thus information recording devices with larger capacity have been required. As a result, there has been a demand for higher recording density of information recording media year by year.
To contribute to satisfying this demand, a perpendicular magnetic recording system has been employed and mass production thereof has been advanced.
In the perpendicular magnetic recording system, heat resistance, surface smoothness, and mechanical strength of substrates are required to a level higher than that of conventional substrates. Young's modulus, flexural strength, and fracture toughness have become especially important evaluation points in mechanical strength, and an improvement in these properties is a problem to be solved.
It is also more important nowadays that specific gravity of a base material should be lowered so as to alleviate load placed on spindle motors or to prevent disks from being damaged when dropped.
Examples of the material used in substrates for information recording media include Al alloy, glass, and crystallized glass. Glass and crystallized glass are superior to Al alloys in view of higher Vickers hardness, Young's modulus, higher surface smoothness, and the like, and thus have often been employed in applications where dynamic use is envisaged.
Crystallized glass is also called glass ceramics and is a material obtained by heating glass to thereby precipitate crystals inside the glass, and is distinguished from amorphous solid. In the case of those produced by using glass as a starting material, those converted completely into crystals may be used as crystallized glass. Crystallized glass can be imparted with physical properties, which cannot be obtained by glass, by virtue of crystals dispersed inside the crystallized glass. Crystallized glass can impart characteristic values regarding mechanical strength such as Young's modulus or fracture toughness, etching resistance to an acidic or alkaline chemical solution, thermal properties such as thermal expansion coefficient, increase and disappearance (improvement in heat resistance) of glass transition temperature and the like, that cannot be realized by glass.
Similarly, crystallized glass can be imparted with physical properties which are different from those of ceramics obtained by sintering powders. Crystallized glass is produced by using glass as a starting material and precipitating crystals inside the glass, and is therefore free from vacancies as compared with ceramics, thus enabling formation of a dense structure.
While crystallized glass in current use is imparted with higher mechanical strength by virtue of crystals precipitated in a glass phase, the crystallized glass exhibits a difference in processing between precipitated crystals and glass phases or a difference in etching rate. Therefore, the crystallized glass currently being employed do not sufficiently satisfy requirements of surface roughness of Ra<2 {acute over (Å)}, which is required of next generation substrates.
Furthermore, since the glass is brittle, defects such as breakage of substrates originating at microcracks on a surface of substrates are likely to occur.
Particularly, with the substrates for information recording medium used for the next generation of hard disks, resistance to crack propagation originating from microcracks on the surface of substrates, i.e., fracture toughness, has become an especially important evaluation point since the rotational speed of magnetic disks tends to be higher along with higher recording density. Therefore, the substrates for information recording medium are required to have higher fracture toughness.
However, the glass substrates currently being employed do not easily satisfy such requirements. Thus it is necessary to subject the substrate to a secondary strengthening treatment, for example, long-term chemical strengthening step so as to improve fracture toughness.
In the production of a glass substrate or a crystallized glass substrate, a direct press process is employed to directly press molten glass for the purpose of producing a disk-shaped substrate at lower cost.
In a glass melting process, an arsenic or antimony component has hitherto been used as a clarifying agent so as to remove bubbles from the molten glass when melting the glass. However, use of the arsenic or antimony component has recently been restricted since these components may exert an adverse influence on human bodies and the environment.
Therefore, use of a substitute component of the arsenic or antimony component as a clarifying agent has been studied. However, there arises a problem that, when the other clarifying agent is used, reboil generates in glass due to impact during direct pressing, and bubbles remain inside the substrate after pressing.
Thus, in order to solve these problems, the present inventors have developed a novel crystallized glass substrate for information recording medium and a patent application has been filed previously (Japanese Patent Application No. 2010-051242). Although this crystallized glass substrate for information recording medium is excellent in mechanical strength and surface smoothness after polishing, a lower specific gravity is required for a substrate for information recording medium of the next generation.
Patent Document 1 discloses a substrate for information recording medium, composed of crystallized glass including a gahnite crystal phase. The substrate has high fracture toughness, but surface roughness after polishing does not satisfy the level required for the substrate for information recording medium of the next generation.
It is also impossible for the substrate for information recording medium to satisfy the production cost required by the market due to lower polishing rate because of excessively high surface hardness, and lower productivity because of requiring longer time for the polishing process.
It is also very difficult for the crystallized glass to control degree of crystallinity and grain size of precipitated crystal since crystals are likely to precipitate rapidly when the raw glass is crystallized through a heat treatment.
Patent Document 2 discloses a glass ceramic containing a spinel type compound as a crystal phase. However, the melting temperature of the glass in Examples thereof is in a high temperature range from 1,500° C. to 1,650° C., and also the highest crystallizing temperature is in a range from 950° C. to 1,000° C., therefore, resulting in poor mass productivity. In addition, surface roughness Ra of the resulting glass ceramic is 5.3 {acute over (Å)}, and thus fail to satisfy the desired surface properties.
[Patent Document 1]Japanese Unexamined Patent Application, Publication No. H07-300340
[Patent Document 2]Japanese Unexamined Patent Application, Publication No. H09-77531